Method and system for measuring the relief of an object

ABSTRACT

A method and a system for measuring the relief of an object are described herein. The system includes a grid projecting for projecting a grid, an image acquisition apparatus that includes a camera, and a computer. Providing a reference object having common elements with the object to measure, the method includes the steps of a) positioning the grid at three different known positions relative to the camera and the common elements; b) for each position of the grid, projecting the grid unto the reference object and, with the camera, taking an image of the reference object to yield three images having values for each pixel of the camera and c) computing the reference object phase for each pixel using the three reference object intensity values for the corresponding pixel. Steps a), b) and c) are repeated by replacing the reference object by the object to be measured. The difference of height between the object to be measured and the reference object for each pixel are then computed by subtracting the reference object phase and the object phase for the corresponding pixel.

FIELD OF THE INVENTION

The present invention generally relates to methods for measuring therelief of an object. More specifically, the present invention isconcerned with the use of such systems and methods to inspect the leadcoplanarity on circuit board.

BACKGROUND OF THE INVENTION

The use of interferometric methods to inspect the surface of an objectfor defects or to measure the relief of an object is well known.Generally stated, these methods consist in generating an interferometricpattern on the surface of the object and then analyzing the resultinginterferometric image (or interferogram) to obtain the relief of theobject. The interferometric image generally includes a series of blackand white fringes.

Interferometric methods that require the use of a laser to generate theinterferometric pattern are called “classic interferometric methods”. Insuch classic methods, the wavelength of the laser and the configurationof the measuring assembly generally determine the period of theresulting interferogram. Classic interferometry methods are generallyused in the visible spectrum to measure height variations in the orderof micron.

However, it has been found difficult to use such method to measureheight variations (relief on a surface showing variations beyond 0.5–1μm when they are implemented in the visible spectrum. Indeed, thedensity of the black and white fringes of the resulting interferogramincreases, causing its analysis to be tedious.

Another drawback of classic interferometric methods is that they requiremeasuring assemblies that are particularly sensitive to noise andvibrations.

Surface inspection methods based on Moiré interferometry allow measuringthe relief of an object in the visible spectrum with accuracy much morethan the accuracy of classic interferometric methods. These methods arebased on the analysis of the frequency beats obtained between 1) a gridpositioned over the object to be measured and its shadow on the object(“Shadow Moiré Techniques”) or 2) the projection of a grid on the objectand another grid positioned between the object and the camera that isused to take a picture of the resulting interferogram (“Projected MoiréTechniques”). In both cases, the frequency beats between two gridsproduce the fringes of the resulting interferogram.

More specifically, the Shadow Moiré technique includes the steps ofpositioning a grid near the object to be measured, providingillumination from a first angle from the plane of the object (forexample 45 degrees) and using a camera, positioned at a second angle(for example 90 degrees from the plane of the object), to take picturesof the interferogram.

Since the distance between the grid and the object varies, thisvariation of height produces a variation in the pattern of theinterferogram. This variation in the pattern can then be analysed toobtain the relief of the object.

A drawback to the use of a Shadow Moiré technique for measuring therelief of an object is that the grid must be positioned very close tothe object in order to yield accurate results, causing restrictions inthe set-up of the measuring assembly.

The Projected Moiré technique is very similar to the Shadow Moirétechnique since the grid, positioned between the camera and the object,has a function similar to the shadow of the grid in the Shadow Moirétechnique. However, a drawback of the Projected Moiré technique is thatit involves many adjustments and therefore creates more risk ofinaccuracy in the results since it requires the positioning and trackingof two grids. Furthermore, the second grid tend to obscure the camera,preventing it from being used simultaneously to take other measurements.

A method and a system to measure the relief of an object free of theabove-mentioned drawbacks of the prior-art are thus desirable.

OBJECTS OF THE INVENTION

An object of the present invention is therefore to provide an improvedmethod and system for measuring the relief of an object.

Another object of the invention is to provide such a system suitable forlead coplanarity inspection.

SUMMARY OF THE INVENTION

More specifically, in accordance with the present invention, there isprovided a method for measuring the relief of an object using a cameraprovided with an array of pixels, the method comprising:

-   -   a) projecting a grid on a reference object; the grid being        located at a first position relative to the camera and to the        reference object;    -   b) taking, with the camera, an image of the reference object        illuminated by the projected grid; the image of the reference        object having intensity values for each pixel;    -   c) repeating steps a) and b) at least two times with the grid        being located at two different known positions relative to the        camera and to the reference object to yield at least three        intensity values for each pixel;    -   d) computing the reference object phase for each pixel using the        at least three reference object intensity values for the        corresponding pixel;    -   e) projecting the grid on the object; the grid being located at        the first position;    -   f) taking with the camera an image of the object illuminated by        the projected grid; the image of the object having intensity        values for each pixel position;    -   g) repeating steps e) and f) at least two times with the grid        being located at the two different positions to yield at least        three intensity values for each pixel;    -   h) computing the object phase for each pixel position using the        at least three object intensity values for the corresponding        pixel; and    -   i) computing the difference of height between the object and the        reference object for each pixel using the reference object phase        and the object phase for the corresponding pixel.

According to another aspect of the present invention, there is provideda system for measuring the relief of an object, the system comprising:

-   -   a grid projecting assembly;    -   an image acquisition apparatus including a camera provided with        an array of pixels;    -   a computer configured for        -   a) receiving from the image acquisition apparatus at least            three images of the projected grid onto the object and at            least three images of the projected grid onto the reference            object; each of the images of the projected grid onto the            object corresponding to a different known position of the            grid; each of the images of the projected grid onto the            reference object corresponding to one of the known positions            of the grid;        -   b) computing the reference object phase for each pixel using            the at least three reference object intensity values for the            corresponding pixel;        -   c) computing the object phase for each pixel using the at            least three object intensity values for the corresponding            pixel; and        -   d) computing the difference of height between the object and            the reference object for each pixel using the reference            object phase and the object phase for the corresponding            pixel.

Other objects, advantages and features of the present invention willbecome more apparent upon reading the following non-restrictivedescription of preferred embodiments thereof, given by way of exampleonly, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings:

FIG. 1 is a schematic view of a system for inspecting the surface of anobject according to an embodiment of the present invention;

FIG. 2 is a schematic view of both the image acquisition apparatus andthe grid projection assembly of FIG. 1;

FIG. 3 is a schematic view illustrating the projection of a grid on anobject;

FIG. 4 is a block diagram of a method for measuring the relief of anobject according to an embodiment of the present invention;

FIG. 5 is an image of a sphere mounted to a board, as taken by thesystem of FIG. 1;

FIG. 6 is an image of the board of FIG. 5, illuminated by the grid;

FIG. 7 is an image computed by the system of FIG. 1, representing thephase of the board of FIG. 6;

FIG. 8 is an image of the sphere of FIG. 5 mounted to the board,illuminated by the grid;

FIG. 9 is an image computed by the system of FIG. 1, representing thephase of the sphere with the board of FIG. 8;

FIG. 10 is an image illustrating the phase variation between the imagesof FIGS. 7 and 9;

FIG. 11 is an image representing the phase variation between a modulecomprising lead balls on a substrate and a reference surface;

FIG. 12 is an image representing the phase of the module of FIG. 11;

FIG. 13 is an image representing the phase variation between the phaseof the image of FIG. 12 and the phase image of a complementary surface;

FIG. 14 is an image representing the phase variation between the phasesof the images of the complementary surface and the reference plane;

FIG. 15 is the image of FIG. 14 after unwrapping.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to FIGS. 1 and 2 of the appended drawings, a system 10 formeasuring the relief of an object, according to an embodiment of thepresent invention, will be described.

The surface inspection system 10 comprises a grid projecting assembly11, an image acquisition apparatus 12, and a computer 14 advantageouslyprovided with a storing device 16, an output device 18 and an inputdevice 20.

Turning now more specifically to FIG. 2 of the appended drawings, thegrid projecting assembly 11 and the image acquisition apparatus 12 willbe described in more detail.

The grid projection assembly 11 includes an illuminating assembly 22, agrid 24 mounted to a movable support 26 and a projector 28.

The illuminating assembly 22 advantageously includes a source of whitelight 34 that is projected through the grid 24. For example, the source34 is the end of an optical fiber (not shown) providing light from awhite light source (not shown). An aspherical lens 36 or any othercondenser is also advantageously used between the source 34 and the grid24. Other light sources may also be used. It is also believed to bewithin the reach of a person skilled in the art to conceive anotherilluminating assembly within the spirit of the present invention.

The configuration of the grid 24 may vary depending on the resolutionthat is required to adequately measure the relief of the object 30. Forexample, it has been found that a ronchi ruling having 250 lines perinch allows to measure lead coplanarity of a circuit board, where aresolution around 1 mm is required.

The grid 24 is advantageously mounted to a moveable support 26 thatallows displacement of the grid 24 in a direction perpendicular (seedouble arrow 40 on FIG. 2) to both the lines on the grid 24 and to thedirection of incidence of the light (dashed line 42 on FIG. 2).

The movable support 26 is actuated by a stepping motor (not shown). Thestepping motor is advantageously controlled by a micro-controller (notshown) triggered by the computer 14. Of course, the stepping motor couldbe directly controlled by the computer 14.

A projector 28, in the form of a 50 mm TV lens, is advantageously usedto project the grid 24 onto the object 38.

The angle θ between the direction of incidence of the light (dashed line42 on FIG. 2) and the line of sight of the image acquisition apparatus12 (dashed line 44 on FIG. 2) may vary depending on the nature of theobject 30 to be measured.

It is believed to be within the reach of a person skilled in the art toposition the illuminating assembly 22, the grid 24 and the gridprojector 28 relative to the object 30 to yield a projected grid havingthe desired pitch p onto the object 30.

For example, a ronchi grid, having a density of 250 lines per inch, witha distance 43 of 22 cm between the object 30 and the projector 28, andfor an angle θ of 30 degrees, provides a projected grid having a 0.5 mmpitch p. Such a pitch is equivalent to a variation of height of about 1mm on the surface of the object 30.

Obviously, the pitch of the projected grid will vary with the pitch ofthe grid 24.

As will be explained hereinbelow, the displacement of the projected grid24 on the object 30 may alternatively be achieved by fixing the positionof the grid 24 and by moving the object 30 and the camera 46 together.

It is to be noted that the system 10 does not require a grid between thecamera 46 and the object 30. This advantage will be discussedhereinbelow.

The image acquisition apparatus 12 includes a camera 46, provided withan array of pixels, and is advantageously in the form of a CCD camera46. Such a camera provides, for example, a resolution of 1300×1024pixels.

The image acquisition apparatus 12 also advantageously includes atelecentric lens 48, advantageously mounted to the camera 46 via anoptional extension tube 50.

The configuration of the image acquisition apparatus 12 and the distancebetween the apparatus 12 and the object 30 determines the field of viewof the image acquisition apparatus 12. Alternatively, a desired field ofview can be achieved without the extension tube 50 by distancing thecamera 46 from the object 30.

The CCD camera can be replaced by a conventional camera when thecomputer 14 is configured to digitize the acquired images.

The computer 14 is advantageously configured to control the displacementof the grid 24, to process the images of the object 30 taken by thecamera 46 and to analyze these images to measure the relief of theobject 30.

The computer 14 is advantageously provided with memory means allowingstoring of the images when they are processed by the computer 14 andtherefore increasing the processing speed.

The storing device 16 can be, for example, a hard drive, a writableCD-ROM drive or other well-known data storing means. It can be directlyconnected to the computer 14, or remotely connected via a computernetwork such as the Internet. According to an embodiment of theinvention, the storing device 16 is used to store both the images takenby the image acquisition apparatus 12, the relief of the object 30 andother intermediary results. Those files can be stored in any format andresolution that can be read by the computer 14.

The output device 20 allows visualization of the images and of the dataproduced by the computer 14, and can take many forms from a displaymonitor to a printing device.

The input device 18 can be a conventional mouse, a keyboard or any otherwell-known input device or combination thereof which allows inputting ofdata and commands into the computer 14.

The storing device 16, the display monitor 18 and the input device 20are all connected to the computer 12 via standard connection means, suchas data cables.

The computer 14 can be a conventional personal computer or any otherdata processing machine that includes a processor, a memory andinput/output ports (not shown). The input/output ports may includenetwork connectivity to transfer the images to and from the storingdevice 16.

Of course, the computer 12 runs software that embodies the method of thepresent invention thereof, as will be described hereinbelow.

It is to be noted that the system 10 includes adjustable support means(not shown) to position the image acquisition apparatus 12 and the gridprojecting assembly 11 relative to each other and to the object 30.Alternatively, other registration means can be used without departingfrom the nature and spirit of the present invention.

Before giving a detail description of a method for measuring the reliefof an object according to an embodiment of the present invention, thegeneral theory underlying such a method will first be described. Sincethis theory is believed to be well known in the art and for concisionpurposes, it will only be briefly described herein.

The intensity I(x,y) for every pixel (x,y) on an interferometric imagemay be described by the following equation:I(x, y)=A(x, y)+B(x, y)·cos(ΔΦ(x, y))  (1)where ΔΦ is the phase variation (or phase modulation), and A and B arecoefficient that can be computed for every pixel.

Knowing the phase variation ΔΦ, the object height distribution (therelief) at every point h(x,y) relative to a reference surface can becomputed using the following equation (see FIG. 3): $\begin{matrix}{{h\left( {x,y} \right)} = \frac{{{\Delta\Phi}\left( {x,y} \right)} \cdot p}{2{\pi \cdot {\tan(\theta)}}}} & (2)\end{matrix}$where p is the grid pitch and θ is the projection angle, as describedhereinabove.

Although the above equation is valid for a parallel projection of thegrid on the object, as illustrated in FIG. 3 (note that the incidenceray 60 from the grid projection are parallel), it is believed to bewithin the reach of a person skilled in the art to use another equationif the grid projection is not parallel.

For example, it has been found with a pinhole projection that the pitchp and the angle θ increase with the distance from the grid on the planof the reference surface (see x on FIG. 3). It has been found that witha first order approximation, variations in p and θ cancel each other outand the Equation 2 remains valid within a certain limit of theparameters.

It is believed within the reach of someone skilled in the art tore-evaluate the relation between the variation of height h(x,y) and thephase ΔΦ, and to make corrections to the relation according to theconfiguration of the system used to measure the relief.

Turning now to FIG. 4 of the appended drawings, a method for measuringthe relief of an object according to an embodiment of the presentinvention will be described in more detail.

Generally stated, the method consists in measuring the relief of anobject 30 using the system 10 by performing the following steps:

-   -   100—positioning the grid 24 at a first position relative to a        reference object;    -   102—projecting the grid 24 on the reference object;    -   104—taking, with the camera 46, an image of the reference object        to gather an intensity value for each pixel of the image;    -   106—repeating steps 100 to 104 at least two times with the grid        positioned at two new different known positions to yield at        least three intensity values for each pixel;    -   108—computing the phase for each pixel using the three intensity        values;    -   110—repeating steps 100 to 108 by replacing the reference object        with the object 30 to be measured;    -   112—computing, for each pixel, the difference of height between        the object 30 and the reference object by using the respecting        phases thereof for every pixel; and    -   114—determining the relief of the object for each pixel using        the difference of height at every pixel.

These general steps will now be further described with reference to afirst example where the object 62 to measure is a sphere 64 mounted to aboard 66. An image of said object 62 can be seen in FIG. 5.

By choosing a similar board as the reference object, the difference ofheight between the object 62 and the reference object will provide theheight of the sphere 64. The common element to the object 62 and thereference object is, in this example, the board 66.

In step 100, the grid 24 is moved to a first predetermined positionusing the support 26 that is actuated by the stepping motor. As it hasbeen discussed hereinabove, the system 10 includes means to register andfix the position of the grid 24 and the camera 46 relative to thereference object (and later the object).

In step 102, the grid 24 is then projected onto the reference object.

In step 104, the camera 46 takes an image of the reference object.

The image includes an intensity value for each pixel of the image. Thecomputer 14 stores these intensity values for future processing.

Steps 100 to 104 are then repeated at least twice with the gridpositioned at two new known different positions (step 106). This willprovide three slightly different images and therefore the threeintensity values for each pixel. One of the three images of the boardilluminated by the grid 24 can be seen in FIG. 6.

Since Equation 1 comprises three unknowns, that is A, B and ΔΦ, threeintensity values I₁, I₂ and I₃ for each pixel, and therefore threeimages, are required to compute the phase variation ΔΦ.

The two new images are taken following small translations of the grid 24relative to the surface of the reference object. The displacements areso chosen as to yield phase variations in the images Δφ₁, Δφ₂ and Δφ₃.This results in three equations similar to Equation 1 for each pixel ofthe pixel array of the camera 46:I _(n) =A+B·cos(ΔΦ+Δφ_(n))  (3)with n=1,3.

By solving the system of Equation 3, one obtains the value of ΔΦ. Thedisplacements of the grid 24 are chosen so as to advantageously providedifferent values of Δφ₁, Δφ₂ and Δφ₃.

According to a preferred embodiment of the present invention, more thanthree images are taken. This yield additional intensity values that canbe used to increase the precision of the calculated phase.

Methods according to the prior-art often require the use of four imagesand all four values from these images are taken for phase estimation.Since a method according to the present invention requires only threeimages, additional images may be used to increase the precision andreliability of the method.

By keeping, for example, four (or more) images, it is possible todiscard noisy pixels or images and to keep only the pixels having themost advantageous intensity values. Indeed, if one of the four intensityvalues is noisy (that can be caused, for example, by an imagesaturation), the corresponding intensity can be eliminated withoutcompromising the precision of the resulting phase for this particularpixel.

Alternatively, more then three intensity values can be used totraditionally compute the phase using a numerical method such as a leastsquare fit. However, such a method could not prevent erroneous phasevalues to be computed for certain pixels, potentially causingimprecision in the computation of the relief of the object.

According to another preferred embodiment of the present invention, thedisplacements of the grid between the second and third images (and thefourth image) are chosen so as to provide two images having 180 degreesphase variations Δφ_(n) (see Equation 3). This allows obtaining an imageof the reference object (or of the object) without the projected grid.This can be achieved by adding the intensity values of the two imagesphase shifted by 180 degrees.

More generally, if the sum of the phase variations of some of the threeor more images taken by the camera 46 is 360 degrees, a correspondingtwo-dimensional image can be obtained by adding the intensity values ofthese images for each pixel. This recomposed two-dimensional image doesnot include the projected grid. This image may be used to perform apreliminary analysis of the reference object (or of the object) that mayspeed-up any subsequent analysis that will be performed on the image orthe values that will result from step 112.

In step 108, the phase is computed using the three intensity values (orthe three best intensity values) for each pixel by solving the Equations3. This can be achieved by using a conventional numerical method, forexample. Numerical methods for solving such system of equation arebelieved to be well known in the art and will not be described herein.

The resulting phase of the reference object for every pixel isillustrated in FIG. 7.

When the method of FIG. 4 is used to inspect a series of objects, steps100 to 108 may be advantageously performed only once for the referenceobject before the inspection. This allows the increase of the speed ofthe inspection.

Steps 100 to 108 are repeated by replacing the reference object by theobject to measure, i.e. the object 62.

One of the images of the sphere 64 with the board 66, illuminated by thegrid 24, can be seen in FIG. 8.

Since there is no difference in performing steps 100 to 108 with theobject and with the reference object, and for concision purposes, thesesteps will not be described again by referring to the object.

The resulting phase of the sphere 64, with the board 66, is illustratedin FIG. 9. It is to be noted that the zone 68 in the image of FIG. 9 iscaused by the shadow of the sphere 64.

In step 112, the difference of height between the object 30 and thereference object is computed for every pixel, as obtained in step 108,by subtracting the phase of the reference object from the phase of theinspected object. The resulting image is shown in FIG. 10.

It is to be noted that the phases computed in step 108 for the objectand for the reference object, and illustrated in FIGS. 7 and 9,correspond to surface phases relative to an imaginary projection plan.

When a non-parallel projection of the grid 24 is done, this imaginaryprojection plan becomes slightly curved. This is not detrimental withthe method for measuring the relief of an object according to thepresent invention since both the images of the object and of thereference object are taken with the same system 10.

Since the phases of the object and of the reference object at each pixelcorrespond to the difference of height between the object (or thereference object) and the same imaginary projection plane (since thesame system with the same optical set-up is used), their subtractionyields the difference of height between the object and the referenceobject. This allows the image acquisition of the object and of thereference object to be performed under different illumination.

In the optional step 114, the relief of the object, i.e. its height, isdetermined for each pixel using the difference of height at every pixelbetween the object and the reference object and knowing the dimensionsof the reference object.

As will now appear obvious to a person of ordinary skills in the art, amethod according to an embodiment of the present invention can be usedto measure the difference of height between two objects (one being thereference). In this case, step 114 is obviously not performed.

In some applications, it may be advantageous to use a plane surface onwhich the object to measure will be laid on during measurement as thereference object.

In some applications, it may be advantageous to provide the system 10with a registration system to help position the object and the referenceobject to a known position relative to the camera. Indeed, since acomparison between the object and the reference object is performed foreach pixel, a registration system may ensure that corresponding pointsare compared.

Such registration system may take many forms including indicia on planesurface, a stand or a software program implemented in the computer.

It is to be noted that the images may be first acquired and thenprocessed at a future time without departing from the spirit of thepresent invention.

As will be apparent upon reading the present description, a method,according to an embodiment of the present invention, allows the measureof the relief of an object using white light.

Although the present invention has been described with an example wherespherical objects are measured, it allows the inspection and measurementof objects having other configurations.

The same object may also act as the reference object when the system 10is used to study the variation in time of an object's relief.

Alternatively, the reference object may be replaced by a computer modelof the object, generated, for example, by a Computer Assisted Design(CAD) that would have been virtually positioned according to the set-upof the system 10.

The reference object may also be a similar object having defects withinacceptable parameters. Hence, the subtraction of the phases of theobject and of the reference object will set forth the defect of theobject under inspection. This aspect of the invention is particularlyinteresting to inspect the relief of an object having importantvariations of relief.

Indeed, since the phase values are limited in the range 0 to 2π themaximum height h₀ that can be detected by most systems of the prior-artis $\begin{matrix}{h_{0} = {\frac{p}{\tan(\theta)}.}} & \left( {{see}\mspace{14mu}{Equation}\mspace{14mu} 2} \right)\end{matrix}$

Usually the unwrapping of phase is done by using a grid having a pitch psufficiently large to ensure that all height variations will be in asingle-phase order (0 to 2π).

A drawback to this is the loss of precision that it implies. Forexample, if the object to be measured is tilted according to the imageacquisition apparatus, the lost of precision may be important.

The following example will illustrate how a method according to thepresent invention allows prevention of the above drawback and relates tolead coplanarity inspection on a circuit board.

FIG. 11 is an image showing the relief of a module 69 comprising aplurality of lead balls 70 on a substrate 72. The image of FIG. 11 isobtained by performing steps 110 to 114 of FIG. 4. In this example, theobject is the module 69 (including the substrate 72 and the lead balls70) and the reference object is a reference plane surface (not shown).

It can be seen in FIG. 11, by the variation in the grey shade in theimage, that the substrate 72 is not parallel to a plane surface.Therefore, such image provides less precision in measuring the height ofthe object than if the substrate would have been plane. Indeed, it is tobe noted that the tilt in the substrate 72 on the image is not caused bythe system 12, but reflects the actual configuration of the substrate72. The small variation in height of each lead ball 70 may be lost inthe overall variation in the substrate 72 profile.

Although one can conceive a computer algorithm to virtually rectify thesubstrate on the image, such algorithm may add to the inspection processtime. This can be seen as a drawback when the inspection is performed inreal-time on a production line.

The proposed solution is to use an approximation of the surface of thesubstrate as a second reference object.

Indeed, it may be advantageous, at each pixel, to first find the heightof the substrate 72 relative to a plane surface, secondly the height ofthe lead balls 70 relative to the substrate 72 and to finally add thesetwo heights to provide the overall height of the object, i.e. thesubstrate with the balls.

The phase of the module is illustrated in FIG. 12 and is obtainedthrough steps 100 to 108 of the method of FIG. 4.

Information about the surface of the substrate 72 is then obtained byanalyzing the pixel corresponding to the substrate 72 (between the balls70) on the image of FIG. 12 where a pseudo-phase image of acomplementary surface is computed.

The height of the balls 70 is computed for each pixel (step 112) bysubtracting the phase of the module (FIG. 12), and the phase of thecomplementary surface. The resulting image can be seen in FIG. 13.

Similarly, the height of the substrate 72 is computed for each pixel(step 112) by subtracting the phase of the complementary surface and thephase of the reference plane. The resulting image can be seen in FIG.14. This phase image is then unwrapped (see FIG. 15).

The height of the module 69 is then obtained by adding the height of thephases of FIGS. 13 and 15.

Although the present invention has been described hereinabove by way ofpreferred embodiments thereof, it can be modified without departing fromthe spirit and nature of the subject invention, as defined in theappended claims.

1. A method for measuring the relief of an object using a cameraprovided with an array of pixels, said method comprising the steps of:a) projecting a grid on a reference object; the grid being located at afirst position relative to the camera and to the reference object; b)taking, with the camera, an image of the reference object illuminated bysaid projected grid; said image of the reference object having intensityvalues for each pixel; c) repeating steps a) and b) at least two timeswith the grid being located at two different known positions relative tothe camera and to the reference object to yield at least three intensityvalues for each pixel; d) computing the reference object phase for eachpixel using the at least three reference object intensity values for thecorresponding pixel; e) projecting the grid on the object; the gridbeing located at said first position; f) taking with the camera an imageof the object illuminated by said projected grid; said image of theobject having intensity values for each pixel position; g) repeatingsteps e) and f) at least two times with the grid being located at saidtwo different positions to yield at least three intensity values foreach pixel; h) computing the object phase for each pixel position usingthe at least three object intensity values for the corresponding pixel;and i) computing the difference of height between the object and thereference object for each pixel using said reference object phase andsaid object phase for the corresponding pixel; and j) using saiddifference of heights between the object and the reference object foreach said pixel to determine the relief of the object.
 2. A method asrecited in claim 1, wherein, in at least one of steps d) and h), thephase ΔΦ is computed for each pixel by solving the following system ofequations:I _(n) =A+B·cos(ΔΦ+Δφ_(n)) where I_(n) represent the at least threeintensity values, A and B are known coefficients and Δφ_(n) are phasevariations caused by the different locations of the grid.
 3. A method asrecited in claim 2, wherein said system of equations is solved using anumerical method.
 4. A method as recited in claim 1, wherein, in stepc), steps a) and b) are repeated more than two times with the grid beinglocated at more the two different known positions relative to the cameraand to the reference object to yield said at least three intensityvalues and at least one additional value for each pixel and, in step d),a selection is performed among the at least three intensity values andthe at least one additional values to yield the three most advantageousintensity values; said three most advantageous intensity values beingused to compute the reference object phase for each pixel.
 5. A methodas recited in claim 4, wherein, in step c), steps a) and b) are repeatedmore than two times with the grid being located at more than twodifferent known positions relative to the camera and to the referenceobject to yield more than tree intensity values and, in step d), thethree most advantageous values from said more than three mostadvantageous intensity values are used to compute the reference objectphase for each pixel.
 6. A method as recited in claim 1, wherein, instep g), steps e) and f) are repeated more than two times with the gridbeing located at more than two different known positions relative to thecamera and to the object to yield said at least three intensity valuesand at least one additional value for each pixel and, in step h), aselection is performed among the at least three intensity values and theat least one additional values to yield the three most advantageousintensity values and said three most advantageous intensity values areused to compute the object phase for each pixel.
 7. A method as recitedin claim 1, wherein, in step g), steps a) and b) are repeated more thantwo times with the grid being located at more than two different knownpositions relative to the camera and to the object to yield more thanthree intensity values and, in step c) the three most advantageousvalues form said more than three intensity values are used to computethe object phase for each pixel.
 8. A method as recited in claim 1,wherein, in step c), said two known positions of the grid are chosen soas to provide at least two images of the object having a 180 degreesdifference in phase therebetween.
 9. A method as recited in claim 8,wherein a two-dimensional image of the object is computed by subtractingsaid at least two images of the object having a 180 degrees differencein phase therebetween; said two dimensional image being used to performa preliminary analysis of the object.
 10. A method as recited in claim1, wherein, in step g), said two known positions of the grid are chosenso as to provide at least two images of the reference object having a180 degrees difference in phase therebetween.
 11. A method as recited inclaim 1, wherein a two-dimensional image of the reference object iscomputed by subtracting said at least two images of the reference objecthaving a 180 degrees difference in phase therebetween; said twodimensional image being used to perform a preliminary analysis of thereference object.
 12. A method as recited in claim 1, wherein saidreference object is a plane surface.
 13. A method as recited in claim 1,wherein said reference object is said object at a past predeterminedtime and said reference object phase is computed around said past time;whereby step i) provides the variation of height at each pixel betweensaid past time and take approximate time when the object phase iscomputed and said step j) yields the variation with time of relief ofthe object.
 14. A method as recited of claim 1, wherein said referenceobject is a CAD of the object; said grid being virtually positioned andprojected into said CAD in step a) and said image of said referenceobject being simulated in step b).
 15. A system for measuring the reliefof an object, said system comprising: a grid projecting assembly; animage acquisition apparatus including a camera provided with an array ofpixels; a computer configured for a) receiving from the imageacquisition apparatus at least three images of the projected grid ontothe object and at least three images of the projected grid onto thereference object; each of said images of the projected grid onto theobject corresponding to a different known position of the grid; each ofsaid images of the projected grid onto the reference objectcorresponding to one of said known positions of the grid; b) computingthe reference object phase for each pixel using the at least threereference object intensity values for the corresponding pixel; c)computing the object phase for each pixel using the at least threeobject intensity values for the corresponding pixel; and d) computingthe difference of height between the object and the reference object foreach pixel using said reference object phase and said object phase forthe corresponding pixel; and e) using said difference of heights betweenthe object and the reference object for each said pixel to determine therelief of the object.
 16. The use of the method of claim 1 forlead-coplanarity inspection.